In recent years, as a solid-state image-capture element such as a CCD (Charge Coupled Device) image sensor, a CMOS (Complementary Metal Oxide Semiconductor) image sensor and the like has been made to have a high resolution, demands for an information device having an imaging function such as a digital still camera, a digital video camera, a mobile phone, a PDA (Personal Digital Assistant) and the like sharply increase. In the meantime, the information device having the image-capture function is referred as an imaging device.
Incidentally, regarding a focusing control method of detecting a distance to a main subject and focusing on the subject, there are a contrast AF method and a phase difference AF (Auto Focus). Since the phase difference AF method can detect a focus position at higher speed and with higher precision, compared to the contrast AF method, it is adopted in various imaging devices.
In the meantime, regarding a method of reading out a signal when imaging a video in an imaging device using the COMS image sensor, a rolling shutter method has been known in which a reset and a readout are sequentially performed from an upper of pixel lines of the CMOS image sensor. In the rolling shutter method, a time difference occurs every pixel line as regards readout timing. For this reason, it has been known that an image of a moving subject is distorted.
Therefore, upon the image-capture of a moving subject with the imaging device using the CMOS image sensor, when a focusing is controlled by the phase difference AF method, an error may be caused in phase difference detection by an image movement or image change occurring while the readout timing deviates, which results from an influence of the distortion caused due to the rolling shutter.
Patent Document 1 discloses a device that, when a focus detection result having reliability is not obtained by focus detection pixels arranged in a horizontal direction, performs focus detection by focus detection pixels arranged in a vertical direction, and when movement of a subject is detected, does not perform the focus detection by the focus detection pixels arranged in the vertical direction.
Also, Patent Documents 2 and 3 disclose a device performing control so that charge accumulation timing of pixels for phase difference detection is the same.    Patent Document 1: Japanese Patent Application Publication No. 2009-128579A    Patent Document 2: Japanese Patent Application Publication No. 2008-72470A    Patent Document 3: Japanese Patent Application Publication No. 2008-263352A
However, according to the technology disclosed in Patent Document 1, it is not possible to prevent false detection of the phase difference caused due to the rolling shutter. Also, according to the technologies disclosed in Patent Documents 2 and 3, since an additional circuit is required, the cost is increased.
The invention has been made keeping in mind the above situations, and an object of the invention is to provide an imaging device and a focusing control method thereof capable of detecting a phase difference with high precision and performing focusing control with high precision by reducing an influence of distortion caused due to a rolling shutter, without an additional circuit.